Electrical circuit inspection systems are well known, and include, for example, INSPIRE-9060™ and SK-75™ automated optical inspection systems available from Orbotech Ltd. of Yavne, Israel. These electrical circuit inspection systems employ multiple inspection channels.
After acquiring a gray level image, or a color image, of an electrical circuit to be inspected, in a first inspection channel an enhanced resolution binary image of the electrical circuit is generated from the acquired image and subsequently is analyzed to locate possible defects in the electrical circuit.
In parallel, in a second inspection channel, the gray level image is directly analyzed, typically to detect relatively small defects such as pinholes, copper splashes and fine short circuits. Analysis of the gray level image typically is performed with reference to various gray level characteristics of an image portion. The characteristics include a change in gray level values, such as a rise and/or a fall in gray level values, along various axes in the image portion. Very often the defects which are to be detected by gray level analysis are not readily visible to the naked eye, simply by human inspection of the gray level image. One reason for this difficulty is that the change in gray level values may appear insignificant to the naked eye, and the possible number of permutations of changes in gray level values, to be representative of defects or non-defects, can be relatively large.